How high should a flat roof upstand be? AccuRoof’s Design and Technical Manager Daniel Bosworth highlights best practice for upstand design in the light of the revised BS 6229.
One of the most common questions our specification managers are asked is, ‘what should the upstand height be on a flat roof?’. This question is frequently followed by, ‘what if I haven’t got 150mm?’. This article is about why upstands exist and how you can avoid asking the second question.
Why do we need flat roof upstands?
The requirement for flat roof upstands is set out in the latest revision to the British Standard: BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings (Section 4.5). The code requires that:
“At all abutments the waterproof layer should be turned up to a level not less than 150 mm above the adjacent finished roof system. In the case of protected roofing systems, i.e. paving slabs, gravel, green, etc., that is 150 mm above the upper surface of the protective layer. In exposed retained or attenuated water systems, it should be above the maximum height of the retained or attenuated water. The design of drainage falls should ensure that the continuity of the waterproof covering is maintained for a vertical height of 150 mm above the finished roof level at all abutments, door openings and parapets.”
Upstands are there to deal with the fact that rainwater can build up on a roof during heavy storm events. If you just took your waterproofing up to the edge of the roof and didn’t have an upstand, the local construction would get saturated. If you were interfacing with rainscreen cladding without an upstand and overlap, water will just run into the building.
An upstand, be it against a parapet, penetration or facade, ensures that the waterproofing does its job and that exposed brickwork, external facades or other building elements are not flooded or saturated. Upstands also help protect against rainwater bouncing up off the finished roof surface. There is no great reasoning in the documentation as to why the rule is 150mm – it has probably been adopted from previous standards over the generations.
The upstand question cannot be looked at in isolation, we need to consider the whole situation.
How to deal with level thresholds and exceptions to the 150mm rule
We are often asked, “How do I deal with level thresholds, when IKO & AccuRoof state 150mm upstand minimum?”
AccuRoof has long advocated the adoption of the NHBC relaxation of upstand heights at door thresholds and as detailed within the SPRA Design Guide and this relaxation has now been adopted and is contained within the new BS 6229:2018. This isn’t just a straight forward relaxation from 150 mm to the localised 75mm over the width of the door opening, there are caveats and provisos which must be adhered to.
Some of these requirements do differ slightly between the new BS 6229:2018 and the advice given in the SPRA Design Guide; notably the inclusion of a proprietary drainage channel in front of the threshold which SPRA identify as required while BS 6229:2018 advises it as an option, however the majority of the guidance is identical.
There are now three examples of acceptable door threshold detailing for different construction types. Warm, Inverted and uninsulated are all covered by the standard identifying the various additional requirements that must be included such as:
- Falls to be away from the threshold.
- The provision of overflows if the area has tall upstands or parapets surrounding it.
- The waterproofing rises back to 150 mm above finishes immediately adjacent to the opening.
- A minimum projection for the cill.
At a door threshold, the standard acknowledges that locally a 75mm upstand height is acceptable, but either side must step up, and drainage must fall away from the ‘pinch point’. Note also the requirement for an overflow if the waterproofing covers an enclosed area with an upstand all around, such as on a balcony or roof terrace with a parapet. The overflow must be a minimum of 25mm below the top of the 150mm upstand height.
What if you don’t have 150mm for a flat roof upstand?
If your design does not allow for 150mm upstand in all cases where the exceptions don’t apply, you run the risk of falling foul of building control or third-party insurers such as FM or NHBC as well as non-compliance with the updated British Standard. If the roof doesn’t comply there can be an issue obtaining mortgages, for example. Here are some examples of how to deal with the situation if you find yourself without the requisite 150mm.
1. Introduce mitigation
You may be able to introduce mitigation into your design, such as with localised drop gutters, but if you eat into the insulation to achieve this you may create a cold spot with condensation issues, or you might not achieve the required U-values in some areas. It is a compromise to rely on average U-values and they should not be putting your design at risk.
2. Change the insulation type
It may be possible to change the insulation type to one with a better U-value (for example changing tissue-faced to foil-faced) for a better performing insulant. But you do need to be aware of the knock-on effects. For example, say you’d decided to use a bonded membrane, which traditionally uses tissue-faced insulation, designed at the correct falls (designed at 1:40 to achieve 1:80 everywhere) which resulted in a pinch point where the upstand height available is then only 140mm. This would be non-compliant.
We could look at changing the insulation from tissue-faced to foil-faced, because it has the better U-value. However, most membranes don’t bond to foil faced insulation, (though some do – talk to us) they only bond to tissue-faced, so you’d have to change the method of attaching the membrane from bonded to mechanically-fixed. Remember to use thermally broken tubes for fixing.
3. Change the insulation arrangement
Another alternative to overcoming a shortage in the 150mm required in your design for the upstand heights we could consider an under and over insulation solution, ie to move part of your insulation to below the Vapour Control Layer, where possible. But you do need to be careful that you do a full U-value and interstitial condensation risk analysis to ensure that moving some insulation to gain height will not create an interstitial condensation risk. Make sure you do the sums so as to not create another even worse problem from your solution.
4. Convert the flat roof upstand into a raincheck
If you have a roof at the required falls and an interface at a door threshold compliant with the new Standard, but at two sides you only have the ability to have 75mm (for example due to planning), what to do then? You could take the waterproofing up 75mm, right across the cavity and have a drip across the parapet. By going continuously over the parapet and into a drip, the upstand is only acting as a raincheck.
These examples show that you need an all-round knowledge of roof design to know what can be done with layers and levels and products to help you achieve compliance in any particular situation.
Most flat roof upstand problems arise from restrictions in height, such as planning restrictions, which cannot be avoided. They often arise because the roof is considered too late in the process, when decisions about the rest of the design cannot be changed (or are already built). Imagine if we could design a building from the 150mm upstands down, rather than up from the foundations? In some cases, if we think about the requirements of the roof in time, we may be able to do something incredibly simple, such as knock 20mm off the ceiling height, to give the required 150mm.
Think the design through completely and compliantly, so that when you get to the top of your project you don’t run out of space.
This article originally appeared in Architecture Today.
